Some embodiments relate to shifting apparatus for selectively engaging a drive member, for instance to implement a gear ratio change inside a gearbox.
WO2014/049317 A1 describes a gearbox which is clutchless in the sense that once a car having such a gearbox is in motion, different gear ratios can be selected without dis-engaging a clutch. The passage in WO2014/049317 A1 between page 12, line 17 and page 15, line 32 describes a shifting mechanism for implementing gear ratio changes in such a gearbox. It has been found that in practice a greater torque than expected is needed to rotate a shift shaft to implement gear ratio changes. In some cases where a steep angle on the ramp in the shift barrel is used the mechanism can jam and so prevent the mechanism functioning correctly. Referring to FIG. 8 herein (a reproduction of FIG. 8 in WO2014/049317 A1) this has been attributed mainly to friction between the pins 21 and edges of the slots 22 in the shift shaft 9 through which they extend.
More specifically, sometimes a selector drum of the kind denoted 11 in FIG. 8 is urged along the length of the shift shaft 9. This occurs for instance in the situations mentioned in WO2014/049317 A1 between page 16, line 31 and page 17, line 1 and also between page 18, lines 25 to 32. In the first situation for example, to implement an upshift in gear ratio both torque and an axial force are exerted on a selector drum 11b associated with the lower gear, thereby causing it to move along the shift shaft 9 until the higher gear is engaged. As the selector drum 11b is being forced along the shift shaft 9 the aforementioned pins 21 are dragged against edges of the slots 22 in the shift shaft 9. Sliding friction thus provides that a larger torque than expected is required to be exerted on the shift shaft 9 to cause the selector drum 11b to be forced along its length during an upshift in gear ratio. A similar problem with friction occurs during a downshift whilst positive driving torque is present.